Semicircular Canal

A semicircular canal is one of three semicircular, interconnected tubes located inside each ear. The three canals are:

• the horizontal semicircular canal (also known as the lateral semicircular canal),
• superior semicircular canal (also known as the anterior semicircular canal),
• and the posterior semicircular canal.

The semicircular ducts provide sensory input for experiences of rotary movements. They are oriented along the pitch, roll, and yaw axes.

The anterior and posterior semicircular ducts are oriented vertically at right angles to each other. The lateral semicircular duct is about 30-degree angle from the horizontal plane. The orientations of the ducts cause a different duct to be stimulated by rotation of the head in different planes. Thus, the horizontal canal detects horizontal head movements (such as when you spin in a rotating chair), while the superior and posterior canals detect vertical head movements (such as when you bend forward to pick something up from the floor).

The semicircular canals are a component of the bony labyrinth. Among species of mammals, the size of the semicircular canals is correlated with their type of locomotion. Specifically, species that are agile and have fast, jerky locomotion have larger canals relative to their body size than those that move more cautiously.

Each canal is filled with a fluid called endolymph and contains motion sensors within the fluids. At the base of each canal, the bony region of the canal is enlarged which opens into the utricle and has a dilated sac at one end called the ampulla. Within the ampulla is a mound of hair cells and supporting cells called crista ampullaris. These hair cells are composed of many cilia and embedded in a gelatinous structure called the cupula. As the head rotates the duct moves but the endolymph lags behind. This deflects the cupula and bends the cilia within. The bending of these cilia alters an electric signal that is transmitted to the brain. Within approximately 25-30 seconds of constant motion, the endolymph catches up to the movement of the duct and the cupula is no longer affected, stopping the sensation of acceleration.

This adjustment period is in part the cause of an illusion known as "the leans" often experienced by pilots. As a pilot enters a turn, hair cells in the semicircular canals are stimulated, telling the brain that the aircraft, and the pilot, are no longer moving in a straight line but rather making a banked turn. If the pilot were to sustain a constant rate turn, the endolymph would eventually catch up to the ducts and cease to deflect the cupula. The pilot would no longer feel as if the aircraft was in a turn. As the pilot exits the turn however, the semicircular canals are stimulated to make the pilot think that they are now turning in the opposite direction rather than flying straight and level. In response to this, the pilot will often lean in the direction of the original turn in an attempt to compensate for this illusion. A more serious form of this is called a graveyard spiral. Rather than the pilot leaning in the direction of the original turn, they may actually reenter the turn. As the endolymph stabilizes, the semicircular canals stop registering the gradual turn and the aircraft slowly loses altitude until impacting with the ground.

Findings from a 2009 study by Tanya T. Whitfield demonstrated a critical late role for bmp2b in the morphogenesis of semicircular canals in the zebrafish inner ear. It is suspected that the role of Bmp2 in semicircular canal duct outgrowth is likely to be conserved between different vertebrate species.